The ismm (an iplayer with the DStwo kernel ported to it) does also have a GBA emulator (albeit a somewhat older version than the current DStwo offerings and thus somewhat inferior but still plenty playable for most games), as does the iplayer I think. Whether you can run one on an updated DSi, or indeed DSi without flash cart checks removed I am not sure about as I have not kept up with the DSi side of things, I know a hacked 3ds can boot blocked DS slot carts though. If you can still find one they tend to go for peanuts where DStwos tend not to.
Those two plus the dstwo family are the only "enhanced" flash carts on the market and as such the only ones that do worthwhile emulation of the GBA from the DS slot. The software only options for more conventional DS flash carts are great examples of programming but are not anything I would suggest for experiencing the DS library.
In any case I would probably still suggest something with a GBA slot and a GBA flash cart as you can still do that for not a lot. Failing that one of the DStwo family is probably the better choice.